1. Field of the Invention
The invention relates to spacecraft fitted with heat radiators designed to dissipate the thermal energy generated on board the craft into space.
It relates more especially to spacecraft designed as satellites placed into a geosynchronous orbit, and notably into a geostationary orbit, stabilized so that a given axis of the satellite stays directed toward the Earth.
This latter case is that of telecommunications satellites which have main antennas designed to stay precisely oriented toward a region of the Earth when the satellite is in position.
The electrical power required for the payload of spacecraft, and of geostationary satellites in particular, is supplied by one or more solar panels which can be oriented about an axis in order to keep them pointed toward the Sun. For geostationary satellites, this axis of rotation of the solar panels, denoted Y-axis, is oriented in a north-south direction.
2. Description of the Related Art
Conventionally, geostationary satellites of this type generally have a design of the type shown in FIG. 1.
The body 10 of the satellite contains the propellant tanks, the payload, the equipment bay and the on-board electronics.
Three axes Z, Y and X associated with the satellite are designed to be oriented toward the Earth, in the north-south direction and in the east-west direction, respectively, when the satellite is in position. Generally speaking, the body of the satellite has the shape of a parallelepiped with north and south faces, a face oriented toward the Earth and a face away from the Earth, often called rear face.
The solar panels 12 are mounted on the north and south faces and are orientable about the Y axis.
The main telecommunications antennas 14 are mounted laterally on either side of the satellite, far enough away from the east and west faces (or from only one of them) to clear their transmission or reception lobe. These main antennas must stay precisely pointing toward the regions of the Earth to be covered owing to their narrow lobe.
For maintaining the satellite in position, the body 10 carries low-thrust nozzles 16, supplied with propellants, a certain number of which are situated at the opposite end of the satellite from the Earth. On recent satellites, these nozzles are complemented by one or more plasma thruster systems 18. In operation, the nozzles or generators generate jets or plumes 20 and 22 of hot gas at high velocity. The impact of these jets on surfaces causes undesirable forces, reduces the efficiency of the nozzles or ion generator and could cause damage to the surfaces.
Conventionally, fixed radiating surfaces 24 are mounted on the north and south faces of the satellite and are thermally connected to the satellite loads that generate heat, for example by heat pipes. The increase in thermal power generated on the more recent high-power satellites makes the thermal dissipation surface area of the fixed radiating surfaces inadequate. Consequently, satellites comprising in addition deployable radiators have been designed, which are held against the body of the satellite during launch and which are later deployed, generally by flipping them, to bring them into an orientation toward the north (and/or south). The configurations to be implemented have been designed notably such that, in the deployed state, the radiators do not obstruct the lobe of the main telecommunications antennas. In particular, a satellite has been proposed (WO 99/19212) whose deployable radiators can be flipped around a hinge starting from a position where they are stored against the north or south face into a position where they are situated at the end away from the Earth (in the −Z direction).
This disposition avoids them interfering with the operation of the main telecommunications antennas. In addition, by placing the hinges obliquely, the radiators 25 can be brought into a divergent configuration which significantly removes them from the impact of high-velocity hot gases which could damage them.
This disposition presents a serious limitation because it does not take other issues into consideration. For recovering the control of a satellite that has lost its nominal orientation, for example owing to a navigation error or a fault, it is not possible to use communications via the main telecommunications antennas 14, since they have a very narrow lobe. Consequently, it is usual to place an omnidirectional antenna 26 (in other words, which has a field of view of at least 2π steradians) oriented in the −Z direction (nominally, away from the Earth) and often also to place an antenna 26 in the +Z direction. In the latter case, each of the omnidirectional anntenas covers half of space and together they allow communication between the Earth and the satellite to be maintained, whatever the orientation of the latter.
However, in order to achieve this result, the field of view of the omnidirectional antennas must not be obscured, otherwise there may be blind regions in the coverage of the antenna which could result in loss of contact with the satellite.